1. Field of the Invention
The present invention relates generally to precision optical inspection methods for specimens such as semiconductor wafers, and more specifically to a method and apparatus for performing microscopic inspection and measurement of integrated circuit wafer geometries using laser confocal microscopy.
2. Description of the Related Art
In integrated circuit inspection, particularly inspection of semiconductor wafers or photomasks from which such circuits are fabricated, different methods have been employed to address particular characteristics of the wafers or advantages afforded by specific inspection technologies.
One such method that has previously been employed in semiconductor wafer or photomask inspection is confocal microscopy. Confocal imaging entails suppressing out of focus specimen elements at image formation. The suppression of out of focus elements occurs partially as a result of the specimen not being illuminated and imaged as a whole at one time, but as one point after another, and also due to the detection pinhole, or spatial filter, interposed between the source and specimen. The sequential point imaging in confocal microscopy is obtained using an arrangement of diaphragms which act as both a point source and a point detector simultaneously at optically coexistant points of the path of light rays used to inspect the specimen. Rays which are out of focus are suppressed by the detection pinhole.
Other inspection techniques have been employed with varying results. Non-confocal imaging tends to be highly sensitive to signals located outside the focal plane, which can add unwanted noise to the imaging process. Noise may also result from defect detection on a semiconductor wafer due to the various layers present on the wafer. Confocal imaging using light elements other than lasers have been employed, but such arrangements create illumination spots with inefficient and slow pinholes since the arc or filament cannot increase in intensity, due to limited power.
Laser confocal imaging addresses the drawbacks of these previous systems. An illustration of a typical laser confocal microscopy inspection arrangement for imaging a single point is illustrated in FIG. 1. Laser 101 emits a beam of light rays 102 which passes through a focusing lens 103 and subsequently through first spatial filter 104. After passing through confocal element 104 the light rays flare outward toward beamsplitter 105. Beamsplitter 105 allows the light rays to pass through and toward objective 106, which focuses the light rays toward the specimen. Light rays reflected from an object at the focal plane 107 pass back toward the objective 106 and beamsplitter 105. Beamsplitter 105 at this point reflects the light rays as illustrated toward second confocal element 108. Objects above or below the focal plane 107 are out of focus and are therefore suppressed. The remaining in focus light rays pass to detector 109.
The advantages of confocal microscopy include the feature that light rays from outside the focal plane are not registered. Confocal imaging can provide true three dimensional data recording, but instead gradually optically removes portions of the specimen as those portions move away from the focal plane. In practice, the elements tend to disappear from the field of view. Stray light tends to be minimized in a confocal arrangement.
The drawbacks associated with the arrangement of FIG. 1 are that the system shown therein only of confocal imaging include a limited field of view, typically a small point on the specimen. Thus scanning an entire specimen would require several passes even for moderately sized specimens. Speed and throughput tend to be of great importance during wafer inspection, and thus confocal techniques have been limited in their application.
Multiple scanning spot systems have been employed to increased inspection speed and throughput. These multiple scanning spot systems utilize mechanical polygon scanning spot laser arrangements to provide increased scanning areas. However, the mechanical polygon scanning techniques tend to be highly unstable and do riot provide necessary fine image alignment for comparison of adjacent features under most circumstances.
Certain confocal systems employ techniques for performing inspection of a wafer or specimen but each system has particular negative aspects. For example, U.S. Pat. No. 5,248,876 to Kerstens, et al., illustrates a confocal imaging system using an opaque mask having a slit and a row of pinpoint sensors or a skewed pattern of isolated pinholes with an array of isolated pinpoint sensors in a matching pattern. The problem with such an arrangement is the sensing of data. The Kerstens sensing arrangement employs an array 116 having isolated pinpoint radiation sensors 114. The problem with such a system is that it is inherently slow and, inefficient in scanning large amounts of data. In particular, the Kerstens system has a very limited dynamic range and can result in obscured or saturated parts of the image under normal inspection speeds.
The Kerstens system also uses a type of autofocus system which uses multiple confocal measurements to determine features on the surface of the system. In particular, the effective focus position of the Kerstens system is a function of the position on the wafer such that the geometry effects the ability of the system to focus on a particular feature and measuring the height of a particular feature.
Other known confocal inspection systems can have problems maintaining focus on a single layer in a multiple layer specimen, such as a CMP (Chemical Mechanical Planarization) specimen. On a multiple layer specimen, inspection of the topmost surface may be required, and certain systems employing confocal techniques do not provide the ability to discriminate or focus on the desired layer. Focusing becomes a problem due to decreasing line widths, desire for increased optical resolution, and a corresponding decrease in depth of focus. Most autofocus systems, such as the autofocus system presented in the Kerstens reference, see through the multiple layers on specimens such as CMP specimens to follow the underlying layers, resulting in non-planar focus performance having varying sensitivity to surface defects resulting from following the underlying topology.
It is therefore an object of the current invention to provide an inspection system which provides noise reduction in images received and the ability to control focus in a multiple depth environment, such as a CMP specimen, in particular with respect to signals which are out of the depth of focus and out of the depth of application interest of the system.
It is a further object of the current invention to provide a high speed and accurate brightfield and darkfield image inspection system without the speed, illumination, and processing drawbacks of traditional laser inspection systems or non-laser systems utilizing lamps.
It is another object of the current invention to provide an inspection system which minimizes the instabilities associated with mechanical scanning systems.
It is yet another object of the current system to provide a confocal system having the ability to inspect a comparatively large area at a relatively rapid rate with minimal distortion at a maximum dynamic range.
It is a further object of the current system to provide for a robust and effective focusing system for multiple layer specimens that is simple and has the ability to effectively discern and account for height differences within the specimen.